This invention relates to the isolation of carboxy-substituted aliphatic hydrocarbons predominantly free from carboxy-substituted aliphatic hydrocarbons having different degrees of carboxy substitution.
Specifically, the invention relates to the isolation, one from the other, of mono-, di-, and tricarboxy-substituted fatty alkyl esters which occur in mixtures with the unsubstituted fatty alkyl esters and other neutral compounds.
The foregoing mixtures may be conventionally obtained by hydroformylation and then oxidation of unsaturated fatty alkyl esters. The unsaturated fatty alkyl esters are obtained by methods well known in the art from vegetable oils such as olive, soybean, safflower, and linseed oil. Vegetable oils are normally mixtures of various unsaturated aliphatic glycerides. Each aliphatic chain can be either mono-, di-, or triunsaturated. For example, the triunsaturated alkyl fatty ester methyl linoleate is found predominantly in transesterified linseed oil.
The unsaturated fatty alkyl esters are first hydroformylated, preferably by the method of Frankel disclosed in U.S. Pat. No. 3,787,459. This method is desirable because it yields products exhibiting little or no isomerization. In this hydroformylation process unsaturated bonds are reacted with hydrogen and carbon monoxide in the presence of a rhodium metal-trisubstituted phosphine catalyst admixture. The resulting product, depending upon the starting material, contains formyl substituents on either, but not both, carbon atoms found at each end of every double bond in the unsaturated fatty alkyl ester.
The formyl groups are then oxidized by the permanganate method disclosed in Frankel, supra, by dichromate (Dufek et al., J. Amer. Oil Chem. Soc. 49: 302-306 [1972] ), or by catalytic reaction with gaseous oxygen (Schwab, U.S. Pat. No. 3,804,895). The formyl substituents are converted to carboxy groups, thus yielding a mixture of acidic fatty alkyl esters with a carboxy group attached to one of the carbons at each of the original sites of unsaturation in the fatty alkyl ester. For example, hydroformylation and oxidation of soybean oil methyl esters yields the following three stearates: ##STR1## Of course, the presence of di-, tri-, or higher carboxy-substituted fatty alkyl esters and the predominance of each ester will depend upon the unsaturation of the starting material.
These mixtures of carboxy-substituted fatty alkyl esters are useful for the same purposes as fatty acids. The carboxy groups then may be esterified by conventional methods (Dufek et al., supra) to yield polyesters valuable as intermediates in the preparation of polyamides used in leather finishing (U.S. Pat. No. 2,957,783); in adhesives, castings, and protective coatings (U.S. Pat. No. 3,062,273), in plasticizing vinyl halide resins (U.S. Pat. No. 2,965,598), and as lubricants (Dufek et al., supra).
The use of these polyester mixtures as high temperature lubricants has been handicapped by the presence of low boiling components. The greater the ester substitution of the aliphatic hydrocarbon the higher the boiling point, so it is highly desirable to provide the most highly substituted hydrocarbon as the principal lubricant component. This can be accomplished by separating the highly substituted compounds before esterification.
Therefore, it is an object of this invention to procure isolates of mono- and polycarboxy-substituted aliphatic hydrocarbons in high yields and at high levels of purity.
It is a further object of this invention to procure these isolates from complex mixtures of such hydrocarbons in a simple and expedient fashion.
I have discovered a process for separating a carboxy-substituted aliphatic hydrocarbon mixture into fractions, each fraction having as its predominant component a carboxy-substituted aliphatic hydrocarbon exhibiting a single degree of carboxy substitution, the process comprising serially extracting a solution of the mixture with aliquots of aqueous alkali, separating each aliquot from the solution, acidifying each aliquot, and recovering the desired carboxy-substituted aliphatic hydrocarbon fraction. By the process of the instant invention the most highly carboxy-substituted aliphatic hydrocarbons are extracted first, followed by components having a lesser degree of carboxy substitution until only neutral hydrocarbons remain. This result was entirely unexpected because those skilled in the art would expect alkali to react without any preference with any acidic component of similar structure, thus extracting all acid components in the same proportion that they occur in the starting mixture.